The Chinese technology company revealed the prototype in a post to the social media site Weibo. It promised the "double-folding" device was coming and asked the public to help name it.
Samsung and the start-up Royole have previously unveiled flexible-screened phones of their own.
But they only folded at a single point.
"It is very interesting that Xiaomi has adopted a three-way folding option on its prototype," said Ben Wood, from the consultancy CCS Insight.
"It is hard to judge the relative benefits of this versus a single fold and it will depend considerably on the user interface that Xiaomi chooses to adopt as it opens the ability to have a three-quarter screen option.
"However, it certainly means there is an additional point of failing. The fold is typically the area where there are most likely to be problems over time."